In recent years, a new class of atomic sensors has emerged based on Rydberg atoms [1]}. Rydberg atoms are atoms from which one or more electrons has been promoted to a highly excited state [2]} and are characterised by their large size, electric polarizability and strong electric dipole transitions between neighboring states. Rydberg atoms are therefore very sensitive to both static and oscillating electric fields. Furthermore, using the technique of electromagnetically induced transparency (EIT) [3]}, [4]} it is possible to achieve coherent optical detection of Rydberg atoms in a thermal vapour [5]} and therefore have a direct readout of the effects of applied electric fields on the atoms. Using Rydberg EIT it has been possible to detect microwave electric fields with unprecedented sensitivity [6]}, [7]}, [8]}, [9]}. This successful technique has subsequently been applied across a huge frequency range spanning from the ultra low frequency [10]} to the terahertz range [11]}.
